1. Field of the Invention
The present invention is directed to methods, devices, and systems for performing refractive surgery on an eye of a patient. In particular, the present invention provides techniques for removing an epithelial layer from a cornea of the eye, especially for reshaping of the cornea underlying the epithelium with a laser.
Ultraviolet and infrared laser based systems and methods are known for enabling ophthalmological surgery on an exposed surface of the cornea in order to correct vision defects. These procedures, generally referred to as photorefractive keratectomy, often employ an ultraviolet or infrared laser to remove a microscopic layer of stromal tissue from the cornea to alter its refractive power. In ultraviolet laser ablation procedures, the radiation ablates corneal tissue by photodecomposition, which does not cause thermal damage to adjacent and underlying tissues. Instead, molecules at the irradiated surface are broken into smaller volatile fragments without heating the remaining substrate. The mechanism of the ablation is photochemical, i.e., the direct breaking of intermolecular bonds. Laser ablation can remove stromal tissue to change the contour for a variety of purposes, including correction of myopia, hyperopia, and astigmatism.
Of particular interest to the present invention, an outer epithelial layer of the cornea is often removed before the stromal tissue is treated. The epithelial layer is typically about 50 .mu.m thick, and removal has commonly been performed using a blunt spatula or other instrument for scraping the layer from the stroma.
Scraping the epithelial layer is disadvantageous in a number of respects. Use of a scraping instrument can impart irregularities to the stromal layer which can adversely effect subsequent laser treatment of the stroma. Similarly, incomplete removal of the epithelial layer may also adversely effect subsequent reshaping of the stroma. Moreover, since the scraping is performed manually, it is usually necessary to remove more of the epithelium from the eye so that the removal area is significantly larger than the area of the eye which will actually be treated by the laser. Such excess removal of epithelial tissue can increase the time necessary for healing. Additionally, use of a scraping instrument presents a small, but finite risk of infection to the patient.
More recently, rotating brushes have been used for removal of the corneal epithelium. A paper authored by Ioannis G. Pallikaris et al. entitled Rotating Brush for Fast Removal of Corneal Epithelium, JOURNAL OF REFRACTIVE AND CORNEAL SURGERY, 10:439 (July/August, 1994), describes the use and structure of a hand held circular rotating brush for removal of the epithelial layer from the central cornea. U.S. Pat. No. 5,649,943, describes a similar device which is fabricated by modifying an electric toothbrush.
These rotating brush structures have been found to remove the epithelial layer more quickly and evenly than scraping, significantly reducing the sensitivity of the epithelial removal process to variations in the surgeon's skill. Nonetheless, the epithelial removal brush structures and methods proposed to date still suffer from significant disadvantages. In general, it is difficult to precisely control the total epithelial removal area when using these known brush techniques. To ensure that adequate access is provided for the photoablative laser, epithelial tissue is removed radially beyond the laser treatment site. As a result, regeneration of the epithelial layer and post-operative healing is delayed. Additionally, the existing brush techniques still rely on the surgeon's skill to ensure that sufficient force is applied to the brush to remove the epithelium in a timely fashion without inadvertently removing stromal tissue.
For all of the above reasons, it would be desirable to provide improved devices, systems, and methods for refractive tissue. It would be particularly desirable to provide enhanced methods for removing the epithelial layer from the cornea so as to provide access to the underlying stroma. It would further be desirable if these improved techniques provided highly reliable and repeatable results without delaying the total refractive therapy.
2. Description of the Background Art
Dr. Pallikaris' article in the JOURNAL OF REFRACTIVE AND CORNEAL SURGERY was described above, together with U.S. Pat. No. 5,649,943. The use and structure of a known rotating brush are also described in an article entitled New PRK Protocol Highly Effective with Few Complications, EUROPEAN SOCIETY OF CATARACT AND REFRACTIVE SURGEONS 2:3 (September/October, 1997), as written by John F. Henahan.
U.S. Pat. No. 4,834,748, describes a method and apparatus for removing corneal tissue. U.S. Pat. No. 5,569,279, describes a surgical abrading device. A method and apparatus for re-profiling the cornea to correct for hyperopia are described in U.S. Pat. No. 5,318,044.
U.S. Pat. No. 5,647,865, describes corneal surgery using a laser, donor corneal tissue, and a synthetic material. U.S. Pat. No. 5,269,795, describes a trephine device for removing anterior epithelial cells from corneal surfaces. A medical treatment of the eye involving removal of the epithelium is described in U.S. Pat. No. 5,312,330. U.S. Pat. Nos. 5,464,417, and 5,100,689, may also be relevant.